Many such field devices are manufactured and sold by the applicant. The process variable to be determined and/or monitored is, for example, the flow of a fluid through a measuring tube, or the fill level of a medium in a container. It can, however, also be pressure, density, viscosity, conductivity, temperature or pH value. Also known are optical sensors, such as turbidity or absorption sensors.
The establishing of a flange connection between a field device and a containment is a common measure for sealed and releasable connection in industry. Correspondingly, usual containments often have a standardized opening with a counterflange, which is usually directly welded onto the containment. Similarly, the flange is often directly welded to the field device.
The terminology, containment, is meant to include both pipelines as well as also containers, for example, a tank. Field devices, which serve for measuring flow, are usually inserted into an existing pipeline by means of two flange connections. If the fill level in a container is to be determined and/or monitored, then, as a rule, only one flange is required, by means of which the measuring device is mounted on a container having a corresponding counterflange. For example, the counterflange can be in or on a nozzle of the container.
The sensor unit and the flange are usually connected with the at least one housing module in such a manner that during operation of the field device the sensor unit can determine and/or monitor the at least one process variable. Arranged between the housing module and the flange can be a neck tube for temperature isolation. Installation of a field device by means of a flange connection makes sense, as a rule, when the sensor unit, due to the applied measuring principle, must at least partially and at least at times contact the medium. Correspondingly, the arrangement of the at least one flange and the sensor unit with the housing module should assure this. In the case of a fill-level measuring device, for example, it is expedient to have the flange surround the sensor unit in the region, in which the sensor unit is connected with the housing module and, in given cases, protrudes partially inwardly into the housing.
Moreover, associated with the apparatus can be an electronics unit, which can be either in the immediate vicinity of the sensor element or spatially separated therefrom. Thus, the electronics unit is accommodated either in the same housing module as the sensor element or in a separate, second housing module.
In the case of a bolted connection of two flanges, the state of sealing of the flange connection correlates with the compressive pressure of the sealing surfaces of the two flanges on a seal arranged therebetween. The compressive pressure often comes from bolts, which extend through bores in the flange plates. Besides a bolted connection, there are, however, also other methods of connecting two flanges. For example, the two flanges can also be directly welded together.
An especially frequently applied material both for containments as well as also for flanges is stainless steel. However, also other metals, such as, for example, tantalum, Hastelloy or composite materials, e.g. carbon steel, are used for specific applications. In practice, it can be that a flange of metal can contribute up to 50% of the total weight of a field device and up to 30% of the total costs. In the case of rarer and/or more expensive materials, such as tantalum, the percentage costs can be even higher.
Besides metals, applied in industry are, among others, ceramics and synthetic materials, e.g. plastics. Synthetic materials distinguish themselves especially by their lesser weight, their chemical durability and, in many applications, their sufficient thermal durability. Moreover, plastic parts are often more cost effective in comparison with metals.
For some time now, also fiber reinforced plastic flanges are known. Besides the savings in weight and costs, which they offer, also an increased strength and stiffness can result. Moreover, one obtains from a flexible process technology the opportunity to configure such parts suitably for loading by targeted application of their anisotropic properties.
The most important requirements for a flange connection involve assuring a sealed and stable connection. Moreover, the flange should in its region facing the medium not influence the process and/or the medium.
Sealing problems can occur, for instance, when the flange and the containment are manufactured of different materials. Due to different coefficients of thermal expansion, large temperature changes can lead to destruction of the structural integrity of the flange connection.
Similarly, different materials of containment and flange in the media-contacting region can show different reactions with the same medium. For example, an electrochemical reaction can occur due to different redox potentials of two materials.
For these reasons, usually selected for the flange is the same material as the containment. Since containments are predominantly of metal, thus also the flange is predominantly made of heavy and partially more expensive metal, although plastics, fiber reinforced plastics or similar materials would be advantageous as regards weight and cost.